Heretofore, there has been a semiconductor device in which a poly-silicon diode serving as an electrostatic breakdown protection element is formed on a substrate where a MOS transistor is formed, in order to protect the MOS transistor from electrostatic discharge (ESD) caused by external static electricity. A semiconductor device of this kind is disclosed in Japanese Patent Application Publication No. 2003-69021.
In the semiconductor device, a gate insulating film having a thickness of 30 to 150 nm is formed on top of a drift region, and a thin field oxide film having the same thickness as that of the gate insulating film is formed on top of an electric field relaxation region. A poly-silicon diode is formed on the field oxide film.
Normally, a poly-silicon diode is formed at the same time when a MOS transistor is formed. In accordance with advancement in performance and miniaturization of MOS transistors, the gate insulating film becomes thinner, and thus a base oxide film of the poly-silicon diode becomes thinner at the same time.
Such a thin base oxide film, however, causes a problem that the poly-silicon diode has a rectification failure due to the influence of the electric potential of the base.
To cope with such a problem, there is a semiconductor device in which a poly-silicon diode is formed on an oxide film thicker than a gate oxide film. A semiconductor device of this kind is disclosed in Japanese Patent Application Publication No. 2003-264289.
In the semiconductor device, a thin oxide film having a thickness of approximately 30 nm is formed on an active region where a MOS transistor is to be formed. Moreover, a thick oxide film having a thickness of approximately 100 nm is formed on a field region where a poly-silicon diode is to be formed. These oxide films are formed by thermally oxidizing a surface of a semiconductor substrate, and further, selectively oxidizing the surface with a nitride film used as a mask.
Next, trenches are formed in the active region, and a gate oxide film is formed on an inner surface of each of the trenches. Thereby, the MOS transistor is formed.
However, the manufacturing of the semiconductor requires a long thermal treatment at a high temperature to form the thick oxide film as a base for the poly-silicon diode, so that the semiconductor substrate may possibly be deformed. Accordingly, there is a concern that such deformation may affect the reliability of the semiconductor device.
Moreover, the thick oxide film on the active region and the thin gate oxide film for the MOS transistor are separately formed. Thus, there is a problem that the number of manufacturing steps increases.
Furthermore, the flatness of the semiconductor substrate is reduced when the thick oxide film is formed on the active region. The reduced flatness brings about disadvantages such as reduction in a margin of focal depth in a subsequent lithography process, which in turn causes misalignment of mask, and reduction in coverage of a thin resist layer. Such disadvantages result in a problem of making miniaturization of the MOS transistor more difficult.